In recent years, a wide spread of IP networks supporting Internet Protocol (IP) has been remarkable. With the wide spread of IP networks, it has been becoming common to convert a voice signal into IP packets (hereinafter, referred to as voice IP packets) by use of a so-called VoIP technology and then to transmit the voice IP packets through an IP network.
For the case of transmitting voice IP packets through the IP networks as described above, there has been disclosed a method for switching communication routes between telephone terminals on the basis of communication quality of the IP networks (for example, a transmission delay time of an IP packet) (For example, Patent Document 1).
In this method, voice IP packets are transmitted by use of RTP (real-time transport protocol), In addition, RTCP (RTP control protocol) is used to control the session of voice IP packets. A receiving side of voice IP packets determines the communication quality of the IP network on the basis of information included in a received RTCP packet, and switches a communication route between telephone terminals in accordance with the determination result.
Patent Document 1: Japanese Patent Application Publication No. 2002-344497 (pp. 7-8, FIGS. 5-7)